C–C Bond Formation via Carbocations in the Methane Conversion Under Non-oxidative Conditions

Abstract

The reactions of methane with any hydrocarbon, e.g., ethylene, are more thermodynamically favorable than the polycondensation of methane under non-oxidative reaction conditions. This chapter focuses on C–C bond formation via the reaction of methane with ethylene or benzene, and the reaction of methane itself using Ag+-exchanged zeolites or superacids as catalysts. The mechanism for the activation of a C–H bond of methane and subsequent C–C bond formation is discussed, along with the role of the catalysts. The unique catalytic properties of Ag+-exchanged zeolites for the reaction of methane with ethylene are emphasized, and compared with superacid catalysis. Ag+-exchanged zeolites activate CH4 to selectively produce +CH3 carbenium ions (methoxy species) and silver-hydride species by the heterolytic dissociation of a C–H bond of methane; the +CH3 carbenium ions attack ethylene to produce propylene, accompanied by the simultaneous formation of hydrogen. The generation of +CH3 carbenium ions over Ag+-exchanged zeolites is supported by the progress of the reaction of methane with benzene. Furthermore, 13C MAS NMR spectra showed the formation of methoxy species when an Ag+-exchanged ZSM-5 zeolite was exposed to CH4. However, superacids generate +C2H5 carbenium ions by the addition of H+ to ethylene; these +C2H5 carbenium ions react with the C–H bonds of methane to produce C3H8. The differences between Ag+- and Zn2+-exchanged zeolites in the activation of lower alkanes including methane are discussed. Ag+-zeolites selectively cleave only the C–H bonds of alkanes, while Zn2+-exchanged zeolites cleave both C–H and C–C bonds.